As will be apparent from German Patent 31 40 188, a double worm extruder can comprise an elongated worm housing in which two mutually meshing worms or screws can be disposed next to one another in respective mutually intersecting cylinder bores so that the interior of the housing has an open figure-eight cross section. The screws can be considered to have elongated screw bodies on which the respective helicals rib, flight or thread is formed, the rib of one screw engaging in the gaps between the terms of the rib of the other screw and vice versa to form a meshing relationship between the screws where the bores overlap or intersect.
The thermoplastic material which is supplied in solid form at one end of the housing to the chamber formed by the intersecting bores, is subjected to high shear forces as the material is displaced by the screws which can be rotated by a motor and gearing at the aforementioned end of the housing in which the screws are journalled. The shear applied to the thermoplastic material can be generated between the meshing parts of the screws as they are rotated in opposite senses and between the screws and the stationary walls of the respective bores and frictionally generates hear in the material to be plastified. Additional heat can be generated by the compression of this material and, if desired, the screws or the housing can be additionally heated.
As a result, the thermoplastic material is thoroughly mixed and melted, i.e. plastified, so that at the opposite end of the double worm extruder, a liquefied product emerges.
The liquefied product may be shaped at this other end of the housing, e.g. in an extrusion die, to form a continuous shaped strand of the thermoplastic material, or it can be fed to an injection unit for producing injection molded articles, or it can be used to produce parisons or some other form of the thermoplastic material enabling it to be blown into blow molded articles or blown into a film. The extruded product can be outputted through a wide-slit nozzle or fed between rolls to produce sheets or films if desired.
Thus the double screw extruder can be utilized as the plastifying device for extrusion presses, injection molding machines and like equipment for the shaping of thermoplastics. For the purpose of the present description, the double worm extruder may be simply referred to as an "extruder" regardless of the shaping ultimately carried out of the plastified material.
An extruder of the type with which the invention is concerned thus can be utilized in all fields in the shaping of plastics and of elastomers such as rubber. It can be used wherever such materials must be plastified with or without additional heat.
The double worm extruders have a significant advantage over single worm extruders in that the outputted material is more homogeneous and uniform.
As noted, the opening through which the material is dispensed in the extruder has a figure-eight cross section, i.e. the bores for the two worms laterally intersect. The bores can be cylindrical and continuous of uniform diameter, although they also can be conical. The screws, correspondingly, can be either cylindrical or conical to conform to the bores.
Since the screws rotate in the housing and the gap or clearance in the screws and the housing must be relatively small, a high squeeze-out pressure of the plastified material can be generated. To minimize wear, the inner surface of the housing and the outer surface of the screws, especially the surfaces of the ribs, can be coated with wear resistant coatings.
Nevertheless, in spite of the presence of such coatings, it has been found that the wear of the inner surface of the housing especially is high with conventional double worm extruders like the extruder described in German Patent 31 40 188.
Indeed, the housing must be resurfaced even after relatively short periods of time. It has been found that the wear is especially pronounced where the plastified material is forced at especially high pressures from gaps between the two screws against the surfaces of the housing.